In railroad signalling the fundamental building block is the rail track circuit. It is used primarily to detect the presence of a train on a section or block of track. Secondarily it provides a means for detecting broken rails within the section, although a broken rail generally cannot be distinguished by a receiver from an occupied track. The basis of the track circuit is using the two rails in a series arrangement with an electrical signal transmitter and an electrical signal receiver. The railway car wheels and axle spanning the rails acts as a shunt between the rails. This shunt path created by the presence of a wheel and axle set causes the transmitted electrical signal to short between the rails at the location of the train. This short blocks the signal being sent by the transmitter to the receiver and is used to detect the presence of a train within the block. Some of the factors in determining the maximum length of track in the block that can be protected by a track circuit includes the leakage paths that occur through the ballast from rail to rail. The leakage path through the ballast is generally considered to be a distributed resistance between the rails. This leakage resistance varies in actual operating conditions subject to moisture, quality of ballast, and other factors which are not in control of the railway signal circuit. Therefore the track circuit must operate over a range of ballast resistance that can normally occur in day-to-day operation of the railroad. It is common to express the maximum length while citing the worse case leakage in the ballast. For example, 12,000 foot at 3 ohms per thousand ballast. Technically the dimensions for this figure should be 0.33 mho per thousand, but it is common in the railway industry to express the term dimensionally as ohms per thousand. When ballast leakage resistance is expressed hereafter in ohms per thousand it will be understood by those skilled in the art to mean mho per thousand. As expressed in this example, 3 ohms per thousand is the minimum ballast; it must be recognized that the circuit must work over the entire range of ballast conditions, i.e., from 3 ohms to infinity. At minimum ballast the transmitted signal is attenuated the most, and the receiver must have adequate sensitivity to insure proper operation although the substantial shunt path that exists through the relatively low resistance offered by the ballast. At infinite ballast the receiver signal strength is at its maximum at the receiver end of the track circuit. When at infinite ballast it is a concern to insure that the railway wheel and axle assembly that creates a shunt path having a resistance normally expressed as 0.06 ohms will be detected. To properly function the track circuit must be capable of detecting this 0.06 ohm shunt from rail-to-rail at any place within the block. Another critical factor in determining the maximum length of a track circuit is the occurrence of a rail break at or near the center of the track circuit when the ballast is at an intermediate leakage condition, i.e., the ballast is between 7 ohms and 15 ohms per thousand. In this situation signal attenuation due to the broken rail is at a minimum and there is a greater likelihood of the break being undetected. One of the factors of high importance in achieving a greater track circuit length is the termination of the transmitter or source end with the lowest possible impedance consistent with meeting shunting sensitivity. With simple D.C. track circuits the practical limit of impedance is about 0.5 ohms in series with the battery or voltage source. At a significantly lower impedance, battery current would be excessive with a train shunting the track at the transmitter end. The battery or other voltage source would in essence have a 0.06 ohm shunt directly across the voltage source. In addition to causing excessively high current demands on the battery or voltage source, a sufficiently high voltage would remain imposed across the rails so that the receiver at the other end could not reliably detect the presence of the train. Such condition is clearly inconsistent with the demands of reliability for the track circuit. Under such conditions one train could be sitting on the end of the block associated with the transmitter and not attenuating sufficient signal from the rails to cause the receiver at the opposite end to detect its presence. The undetected train shunting a low impedance source end would permit the receiver to display an unoccupied block to a train entering the section.
With electronic track circuits wherein the voltage and current from the transmitter can be controlled achieving a source impedance approaching zero ohms while limiting short circuit current is entirely practical. However practical, such a power source would be unusable in a conventional track circuit in which one end of the circuit serves as a source and the other as a receiver. This is for the reasons stated above because a zero or near zero ohm source would be unworkable as the circuit would not properly shunt at the source end. If such a zero source impedance was to be used the track section could be increased significantly while maintaining broken rail detection. Traditional track circuits cannot exploit this advantage because the circuit may not reliably shunt at the transmitter or source end. These limiting conditions have generally required that existing track circuits were limited to a maximum block length of approximately 15,000 feet.
While it has been known to use both a source and a receiver at each end of the track circuit, such sources are still relatively high impedance devices limiting the maximum track circuit length. Such units can act in master and master and satellite modes to communicate between ends of the block but are limited by their impedance to traditional track circuit lengths.